Inductive thermography is a non-destructive testing method, whereby the specimen is slightly heated with a short heating pulse (0.1–1 s) and the temperature change on the surface is recorded with an infrared (IR) camera. Eddy current is induced by means of high frequency (HF) magnetic field in the surface ‘skin’ of the specimen. Since surface cracks disturb the eddy current distribution and the heat diffusion, they become visible in the IR images. Head checks and squats are specific types of damage in railway rails related to rolling contact fatigue (RCF). Inductive thermography can be excellently used to detect head checks and squats on rails, and the method is also applicable for characterizing individual cracks as well as crack networks. Several rail pieces with head checks, with artificial electrical discharge-machining (EDM)-cuts and with a squat defect were inspected using inductive thermography. Aiming towards rail inspection of the track, 1 m long rail pieces were inspected in two different ways: first via a ‘stop-and-go’ technique, through which their subsequent images are merged together into a panorama image, and secondly via scanning during a continuous movement of the rail. The advantages and disadvantages of both methods are compared and analyzed. Special image processing tools were developed to automatically fully characterize the rail defects (average crack angle, distance between cracks and average crack length) in the recorded IR images. Additionally, finite element simulations were used to investigate the effect of the measurement setup and of the crack parameters, in order to optimize the experiments.

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Inductive thermography is a non-destructive technique for detecting surface cracks in electrically conductive materials. In non-magnetic materials with high electrical and thermal conductivity short heating pulse is necessary, otherwise the thermal signal diminishes too quickly. But if with short heating pulse only little amount of heat is induced in the material then the signal-to-noise ratio (SNR) is too low for reliable defect detection. Applying a sequence of short pulses offers a solution. SNR of the phase image increases with the number of pulses and the cracks become detectable. Aluminum and non-magnetic steel samples with artificial cracks have been manufactured and tested. The influence of the number of pulses, the heating power, the pulse duration, and the crack depth on the SNR is investigated for both materials.

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